Method and device for multistate interferometric light modulation

ABSTRACT

A multi-state light modulator comprises a first reflector. A first electrode is positioned at a distance from the first reflector. A second reflector is positioned between the first reflector and the first electrode. The second reflector is movable between an undriven position, a first driven position, and a second driven position, each having a corresponding distance from the first reflector. In one embodiment, the three positions correspond to reflecting white light, being non-reflective, and reflecting a selected color of light. Another embodiment is a method of making the light modulator. Another embodiment is a display including the light modulator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/613,486 filed Sep. 27, 2004, and U.S. Provisional Application No.60/613,499 filed Sep. 27, 2004. Each of the foregoing applications isincorporated in its entirety by reference herein.

BACKGROUND

1. Field

The field of the invention relates to microelectromechanical systems(MEMS).

2. Background

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. An interferometricmodulator may comprise a pair of conductive plates, one or both of whichmay be transparent and/or reflective in whole or part and capable ofrelative motion upon application of an appropriate electrical signal.One plate may comprise a stationary layer deposited on a substrate, theother plate may comprise a metallic membrane separated from thestationary layer by an air gap. Such devices have a wide range ofapplications, and it would be beneficial in the art to utilize and/ormodify the characteristics of these types of devices so that theirfeatures can be exploited in improving existing products and creatingnew products that have not yet been developed.

SUMMARY

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Preferred Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

One embodiment is a light modulator. The light modulator includes afixed reflector comprising an electrically conductive layer and apartially reflective layer. The light modulator further comprises anelectrode positioned at a distance from the fixed reflector and defininga first cavity therebetween. The light modulator further comprises amovable reflector comprising an electrically conductive material. Themovable reflector is positioned between the fixed reflector and theelectrode. The movable reflector is movable between an undrivenposition, a first driven position, and a second driven position. Thefirst driven position is closer to the fixed reflector than is theundriven position and the second driven position is farther from thefixed reflector than is the undriven position.

Another embodiment is a light modulator comprising first reflector, afirst electrode positioned at a distance from the first reflector, and asecond reflector positioned between the first reflector and the firstelectrode. The second reflector is movable between an undriven position,a first driven position, and a second driven position. The first drivenposition is closer to the first reflector than is the undriven positionand the second driven position is farther from the first reflector thanis the undriven position.

Another embodiment is a method of driving a MEMS device comprising afirst electrode, a second electrode, and a movable electrode positionedbetween the first electrode and the second electrode and configured tomove to at least two positions therebetween. The method includesapplying a first voltage potential difference between the firstelectrode and the movable electrode so as to drive the movable electrodeto a position substantially in contact with a dielectric layer, whereinan attractive force is created between the movable electrode and thedielectric layer. The method further includes applying a second voltagepotential difference between the first electrode and the movableelectrode and a third voltage potential difference between the secondelectrode and the movable electrode so as to overcome the attractiveforce between the movable electrode and the dielectric layer and todrive the movable electrode away from the dielectric layer.

Another embodiment is a method of fabricating a multistate lightmodulator. The method includes forming a first reflector. The methodfurther includes forming a first electrode positioned at a distance fromthe first reflector. The method further includes forming a secondreflector positioned between the first reflector and the firstelectrode. The second reflector is made movable between an undrivenposition, a first driven position, and a second driven position, whereinthe first driven position is closer to the first reflector than is theundriven position and wherein the second driven position is farther fromthe first reflector than is the undriven position.

Another embodiment is a display comprising a plurality of displayelements. Each of the display elements includes a first reflectivemember, a first conductive member positioned at a distance from thefirst reflective member, and a second reflective member positionedbetween the first reflective member and the first conductive member. Thesecond reflective member is movable between an undriven position, afirst driven position, and a second driven position. The first drivenposition is closer to the first reflective member than is the undrivenposition and the second driven position is farther from the firstreflective member than is the undriven position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a released position and amovable reflective layer of a second interferometric modulator is in anactuated position.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIG. 6A is a cross section of the device of FIG. 1.

FIG. 6B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 6C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7 is a side cross-sectional view of an exemplary interferometricmodulator that illustrates the spectral characteristics of producedlight.

FIG. 8 is a graphical illustration of reflectivity versus wavelength formirrors of several exemplary interferometric modulators.

FIG. 9 is a chromaticity diagram that illustrates the colors that can beproduced by a color display that includes exemplary sets of red, green,and blue interferometric modulators.

FIG. 10 is a side cross-sectional view of an exemplary multistateinterferometric modulator.

FIG. 11A-11C are side cross-sectional views of another exemplarymultistate interferometric modulator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An interferometric modulator has a reflector which is movable betweenthree positions. In an undriven state of the modulator, the movablemirror is in an undriven position. In a first driven state of themodulator, the movable mirror is deflected toward a fixed mirror to afirst driven position which is closer to the fixed mirror than is theundriven position. In a second driven state of the modulator, themovable mirror is deflected away from the fixed mirror to a seconddriven position which is farther from the fixed mirror than is theundriven position. In one embodiment, the modulator is non-reflective,e.g., black, when the movable mirror is in the undriven position,reflects white light when the movable mirror is in the first drivenposition, and reflects a selected color of light when the movable mirroris in the second driven position. A color display including suchmodulators thus reflects relatively intense white light while having alarge color gamut.

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theinvention may be implemented in any device that is configured to displayan image, whether in motion (e.g., video) or stationary (e.g., stillimage), and whether textual or pictorial. More particularly, it iscontemplated that the invention may be implemented in or associated witha variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as thereleased state, the movable layer is positioned at a relatively largedistance from a fixed partially reflective layer. In the secondposition, the movable layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable and highly reflective layer 14 ais illustrated in a released position at a predetermined distance from afixed partially reflective layer 16 a. In the interferometric modulator12 b on the right, the movable highly reflective layer 14 b isillustrated in an actuated position adjacent to the fixed partiallyreflective layer 16 b.

The fixed layers 16 a, 16 b are electrically conductive, partiallytransparent and partially reflective, and may be fabricated, forexample, by depositing one or more layers each of chromium andindium-tin-oxide onto a transparent substrate 20. The layers arepatterned into parallel strips, and may form row electrodes in a displaydevice as described further below. The movable layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes 16 a, 16 b) deposited on top ofposts 18 and an intervening sacrificial material deposited between theposts 18. When the sacrificial material is etched away, the deformablemetal layers are separated from the fixed metal layers by a defined airgap 19. A highly conductive and reflective material such as aluminum maybe used for the deformable layers, and these strips may form columnelectrodes in a display device.

With no applied voltage, the cavity 19 remains between the layers 14 a,16 a and the deformable layer is in a mechanically relaxed state asillustrated by the pixel 12 a in FIG. 1. However, when a potentialdifference is applied to a selected row and column, the capacitor formedat the intersection of the row and column electrodes at thecorresponding pixel becomes charged, and electrostatic forces pull theelectrodes together. If the voltage is high enough, the movable layer isdeformed and is forced against the fixed layer (a dielectric materialwhich is not illustrated in this Figure may be deposited on the fixedlayer to prevent shorting and control the separation distance) asillustrated by the pixel 12 b on the right in FIG. 1. The behavior isthe same regardless of the polarity of the applied potential difference.In this way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application. FIG. 2is a system block diagram illustrating one embodiment of an electronicdevice that may incorporate aspects of the invention. In the exemplaryembodiment, the electronic device includes a processor 21 which may beany general purpose single- or multi-chip microprocessor such as an ARM,Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051,a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessorsuch as a digital signal processor, microcontroller, or a programmablegate array. As is conventional in the art, the processor 21 may beconfigured to execute one or more software modules. In addition toexecuting an operating system, the processor may be configured toexecute one or more software applications, including a web browser, atelephone application, an email program, or any other softwareapplication.

In one embodiment, the processor 21 is also configured to communicatewith an array controller 22. In one embodiment, the array controller 22includes a row driver circuit 24 and a column driver circuit 26 thatprovide signals to a pixel array 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the released state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not releasecompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the released or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be released areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or releasedpre-existing state. Since each pixel of the interferometric modulator,whether in the actuated or released state, is essentially a capacitorformed by the fixed and moving reflective layers, this stable state canbe held at a voltage within the hysteresis window with almost no powerdissipation. Essentially no current flows into the pixel if the appliedpotential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Releasing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias). As is also illustrated in FIG.4, it will be appreciated that voltages of opposite polarity than thosedescribed above can be used, e.g., actuating a pixel can involve settingthe appropriate column to +V_(bias), and the appropriate row to −ΔV. Inthis embodiment, releasing the pixel is accomplished by setting theappropriate column to −V_(bias), and the appropriate row to the same−ΔV, producing a zero volt potential difference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or released states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and releases the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and release pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thepresent invention.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 6A-6C illustrate three different embodiments of themoving mirror structure. FIG. 6A is a cross section of the embodiment ofFIG. 1, where a strip of metal material 14 is deposited on orthogonallyextending supports 18. In FIG. 6B, the moveable reflective material 14is attached to supports at the corners only, on tethers 32. In FIG. 6C,the moveable reflective material 14 is suspended from a deformable layer34. This embodiment has benefits because the structural design andmaterials used for the reflective material 14 can be optimized withrespect to the optical properties, and the structural design andmaterials used for the deformable layer 34 can be optimized with respectto desired mechanical properties. The production of various types ofinterferometric devices is described in a variety of publisheddocuments, including, for example, U.S. Published Application2004/0051929. A wide variety of well known techniques may be used toproduce the above described structures involving a series of materialdeposition, patterning, and etching steps.

Embodiments of interferometric modulators described above operate in oneof a reflective state, which produces white light, or light of a colordetermined by the distance between the mirrors 14 and 16, or in anon-reflective, e.g., black, state. In other embodiments, for example,embodiments disclosed in U.S. Pat. No. 5,986,796, the movable mirror 14may be positioned at a range of positions relative to the fixed mirror16 to vary the size of the resonant gap 19, and thus the color ofreflected light.

FIG. 7 is a side cross-sectional view of an exemplary interferometricmodulator 12 that illustrates the spectral characteristics of light thatwould be produced by positioning the movable mirror 14 at a range ofpositions 111-115. As discussed above, a potential difference between arow and column electrode causes the movable mirror 14 to deflect. Theexemplary modulator includes a conductive layer 102 of indium-tin-oxide(ITO) acting as a column electrode. In the exemplary modulator, themirror 14 includes the row conductor.

In one embodiment, a dielectric layer 104 of a material such as alumina(Al₂O₃) is positioned over a layer of chrome that forms a reflectivesurface of the mirror 16. As discussed above with reference to FIG. 1,the dielectric layer 104 prevents shorting and controls the separationdistance between the mirrors 14 and 16 when the mirror 14 deflects. Theoptical cavity formed between the mirrors 14 and 16 thus includes thedielectric layer 104. The relative sizes of items in FIG. 7 have beenselected for purposes of conveniently illustrating the modulator 12.Thus, such distances are not to scale and are not intended to berepresentative of any particular embodiment of the modulator 12.

FIG. 8 is a graphical illustration of reflectivity versus wavelength forthe mirrors 16 of several exemplary optical stacks. The horizontal axisrepresents a range of wavelengths of visible light incident on theoptical stacks. The vertical axis represents the reflectivity of theoptical stack as a percentage of incident light at a particularwavelength. In one embodiment, in which the optical stack does notinclude the dielectric layer 104, the reflectivity of the mirror 16formed of a layer of chrome is approximately 75%. An optical stackincluding a dielectric layer 104 comprising a 100 Å layer of aluminaresults in 65% reflectivity and a dielectric layer 104 comprising a 200Å layer of alumina results in 55% reflectivity. As shown, reflectivitydoes not vary according to wavelength in these particular embodiments.Accordingly, by adjusting the thickness of an Al₂O₃ layer, thereflectivity of the mirror 16 can be controlled consistently across thevisible spectrum to allow specific properties of interferometricmodulators 12 to be selected. In certain embodiments, the dielectriclayer 104 is a layer of Al₂O₃, having a thickness in the range of 50-250Å. In other embodiments, the dielectric layer 104 comprises a thin layerof Al₂O₃, having a thickness in the range of 50-100 Å and a layer ofbulk SiO₂, having a thickness in the range of 400-2000 Å.

As discussed above, the modulator 12 includes an optical cavity formedbetween the mirrors 14 and 16. The characteristic distance, or effectiveoptical path length, L, of the optical cavity determines the resonantwavelengths, λ, of the optical cavity and thus of the interferometricmodulator 12. The resonant wavelength, λ, of the interferometricmodulator 12 generally corresponds to the perceived color of lightreflected by the modulator 12. Mathematically, the distance L=½ N λ,where N is an integer. A given resonant wavelength, λ, is thus reflectedby interferometric modulators 12 having distances L of ½λ (N=1), λ(N=2), 3/2λ (N=3), etc. The integer N may be referred to as the order ofinterference of the reflected light. As used herein, the order of amodulator 12 also refers to the order N of light reflected by themodulator 12 when the mirror 14 is in at least one position. Forexample, a first order red interferometric modulator 12 may have adistance L of about 325 nm, corresponding to a wavelength λ of about 650nm. Accordingly, a second order red interferometric modulator 12 mayhave a distance L of about 650 nm. Generally, higher order modulators 12reflect light over a narrower range of wavelengths and thus producecolored light that is more saturated.

Note that in certain embodiments, the distance, L, is substantiallyequal to the distance between the mirrors 14 and 16. Where the spacebetween the mirrors 14 and 16 comprises only a gas (e.g., air) having anindex of refraction of approximately 1, the effective optical pathlength is substantially equal to the distance between the mirrors 14 and16. In embodiments that include the dielectric layer 104, which has anindex of refraction greater than one, the optical cavity is formed tohave the desired optical path length by selecting the distance betweenthe mirrors 14 and 16 and by selecting the thickness and index ofrefraction of the dielectric layer 104, or of any other layers betweenthe mirrors 14 and 16. In one embodiment, the mirror 14 may be deflectedone or more positions within a range of positions to output acorresponding range of colors. For example, the voltage potentialdifference between the row and column electrodes may be adjusted todeflect the mirror 14 to one of a range of positions in relation to themirror 16. In general, the greatest level of control of the position ofthe mirror by adjusting voltage is near the undeflected position of thepath of the mirror 14 (for example, for smaller deflections, such asdeflections within about ⅓rd of the maximum deflection from theundeflected position of the mirror 14).

Each of a particular group of positions 111-115 of the movable mirror 14is denoted in FIG. 7 by a line extending from the fixed mirror 16 to anarrow point indicating the positions 111-115. Thus, the distances111-115 are selected so as to account for the thickness and index ofrefraction of the dielectric layer 104. When the movable mirror 14deflects to each of the positions 111-115, each corresponding to adifferent distance, L, the modulator outputs light to a viewing position101 with a different spectral response that corresponds to differentcolors of incident light being reflected by the modulator 12. Moreover,at position 111, the movable mirror 14 is sufficiently close to thefixed mirror 16, that the effects of interference are negligible andmodulator 12 acts as a mirror that reflects substantially all colors ofincident visible light substantially equally, e.g., as white light. Thebroadband mirror effect is caused because the small distance L is toosmall for optical resonance in the visible band. The mirror 14 thusmerely acts as a reflective surface with respect to visible light.

As the gap is increased to the position 112, the modulator 12 exhibits ashade of gray as the increased gap distance between the mirrors 14 and16 reduces the reflectivity of the mirror 14. At the position 113, thedistance L is such that the cavity operates interferometrically butreflects substantially no visible wavelengths of light because theresonant wavelength is outside the visible range.

As the distance L is increased further, a peak spectral response of themodulator 12 moves into visible wavelengths. Thus, when the movablemirror 14 is at position 114, the modulator 12 reflects blue light. Whenthe movable mirror 14 is at the position 115, the modulator 12 reflectsgreen light. When the movable mirror 14 is at the non-deflected position116, the modulator 12 reflects red light.

In designing a display using interferometric modulators 12, themodulators 12 may be formed so as to increase the color saturation ofreflected light. Saturation refers to the intensity of the hue of colorlight. A highly saturated hue has a vivid, intense color, while a lesssaturated hue appears more muted and grey. For example, a laser, whichproduces a very narrow range of wavelengths, produces highly saturatedlight. Conversely, a typical incandescent light bulb produces whitelight that may have a desaturated red or blue color. In one embodiment,the modulator 12 is formed with a distance L corresponding to higherorder of interference, e.g., 2nd or 3rd order, to increase thesaturation of reflected color light.

An exemplary color display includes red, green, and blue displayelements. Other colors are produced in such a display by varying therelative intensity of light produced by the red, green, and blueelements. Such mixtures of primary colors such as red, green, and blueare perceived by the human eye as other colors. The relative values ofred, green, and blue in such a color system may be referred to astristimulus values in reference to the stimulation of red, green, andblue light sensitive portions of the human eye. In general, the moresaturated the primary colors, the greater the range of colors that canbe produced by the display. In other embodiments, the display mayinclude modulators 12 having sets of colors that define other colorsystems in terms of sets of primary colors other than red, green, andblue.

FIG. 9 is a chromaticity diagram that illustrates the colors that can beproduced by a color display that includes two sets of exemplary red,green, and blue interferometric modulators. The horizontal and verticalaxes define a chromaticity coordinate system on which spectraltristimulus values may be depicted. In particular, points 120 illustratethe color of light reflected by exemplary red, green, and blueinterferometric modulators. White light is indicated by a point 122. Thedistance from each point 120 to the point 122 of white light, e.g., thedistance 124 between the point 122 for white and the point 120 for greenlight, is indicative of the saturation of light produced by thecorresponding modulator 12. The region enclosed by the triangular trace126 corresponds to the range of colors that can be produced by mixingthe light produced at points 120. This range of colors may be referredto as the color gamut of the display.

Points 128 indicate the spectral response of another set of exemplarymodulators 12. As indicated by the smaller distance between the points128 and the white point 122 than between points 120 and point 122, themodulators 12 corresponding to the points 128 produce less saturatedlight that do the modulators 12 corresponding to the points 120. Thetrace 130 indicates the range of colors that can be produced by mixingthe light of points 128. As is shown in FIG. 9, the trace 126 encloses alarger area than does the trace 130, graphically illustrating therelationship between the saturation of the display elements and the sizeof the color gamut of the display.

In a reflective display, white light produced using such saturatedinterferometric modulators tends to have a relatively low intensity to aviewer because only a small range of incident wavelengths is reflectedto form the white light. In contrast, a mirror reflecting broadbandwhite light, e.g., substantially all incident wavelengths, has a greaterintensity because a greater range of incident wavelengths is reflected.Thus, designing reflective displays using combinations of primary colorsto produce white light generally results in a tradeoff between colorsaturation and color gamut and the brightness of white light output bythe display.

FIG. 10 is a side cross-sectional view of an exemplary multistateinterferometric modulator 140 that can produce highly saturated colorlight in one state and relatively intense white light in another state.The exemplary modulator 140 thus decouples color saturation from thebrightness of output white light. The modulator 140 includes a movablemirror 14 that is positioned between two electrodes 102 and 142. Themodulator 140 also includes a second set of posts 18 a that are formedon the opposite side of the mirror 14 as the posts 18.

In certain embodiments, each of the mirrors 14 and 16 may be part of astack of layers defining a reflector or reflective member that performfunctions other than reflecting light. For example, in the exemplarymodulator of FIG. 10, the mirror 14 is formed of one or more layers of aconductive and reflective material such as aluminum. Thus, the mirror 14may also function as a conductor. Similarly, the mirror 16 may be formedof one or more layers of reflective material and one or more layers ofan electrically conductive material so as to perform the functions ofthe electrode 102. Furthermore, each of the mirrors 14 and 16 may alsoinclude one or more layers having other functions, such as to controlthe mechanical properties affecting deflection of the mirror 14. In oneembodiment, the moveable mirror 14 is suspended from an additionaldeformable layer such is described in connection with FIG. 6C.

In one embodiment that includes modulators that reflect red, green, andblue light, different reflective materials are used for modulators thatreflect different colors so as to improve the spectral response of suchmodulators 12. For example, the movable mirror 14 may include gold inthe modulators 12 configured to reflect red light.

In one embodiment, dielectric layers 144 may be positioned on eitherside of the conductor 142. The dielectric layers 144 a and 104advantageously prevent electrical shorts between conductive portions ofthe mirror 14 and other portions of the modulator 140. In oneembodiment, the mirror 16 and the electrode 102 collectively form areflective member.

In the exemplary embodiment, the distance between fixed mirror 16 andthe movable mirror 14 in its undriven position corresponds to theoptical path length L in which the modulator 140 is non-reflective or“black.” In the exemplary embodiment, the optical path length betweenthe fixed mirror 16 and the movable mirror 14 when driven towards thefixed mirror 16 corresponds to the optical path length L in which themodulator 140 reflects white light. In the exemplary embodiment, thedistance between the fixed mirror 16 and the movable mirror 14 whendriven towards the conductor 142 corresponds to the optical path lengthL in which the modulator 140 reflects light of a color such as red,blue, or green. In certain embodiments, the distance between theundriven movable mirror 14 and the fixed mirror 16 is substantiallyequal to the distance between the undriven movable mirror 14 and theelectrode 142. Such embodiments may be considered to be two modulatorspositioned around the single movable mirror 14.

When a first voltage potential difference is applied between the mirror14 and the electrode 102, the mirror 14 deflects towards the mirror 16to define a first optical path length, L, that corresponds to a firstdriven state. In this first driven state, the movable mirror 14 iscloser to the mirror 16 than in the undriven state. When a secondvoltage potential difference is applied between the mirror 14 and theelectrode 142, the mirror 14 is deflected away from the mirror 16 todefine a second optical path length, L, that corresponds to a seconddriven state. In this second driven state, the movable mirror 14 isfarther from the mirror 16 than in the undriven state. In certainembodiments, at least one of the first driven state and second drivenstate is achieved by applying voltage potential differences both betweenthe mirror 14 and the electrode 102 and between the mirror 14 and theelectrode 142. In certain embodiments, the second voltage difference isselected to provide a desired deflection of the mirror 14.

As illustrated in FIG. 10, in the first driven state, the mirror 14deflects to a position indicated by the dashed line 152. In theexemplary modulator 140, the distance between the mirrors 14 and 16 inthis first driven state corresponds to the thickness of the dielectriclayer 104. In the exemplary modulator 140, the mirror 14 acts as abroadband mirror in this position, substantially reflecting all visiblewavelengths of light. As such, the modulator 140 produces a broadbandwhite light when illuminated by broadband white light.

In the second driven state, the mirror 14 deflects to a positionindicated by the dashed line 154. In the exemplary modulator 140, thisdistance corresponds to a color of light, e.g., blue light. In theundriven state, the mirror 14 is positioned as shown in FIG. 10. In theundeflected position, the mirror 14 is spaced at a distance from themirror 16 so that substantially no visible light is reflected, e.g., an“off” or non-reflective state. Thus, the modulator 140 defines aninterferometric modulator having at least three discrete states. Inother embodiments, the positions of the movable mirror 14 in the threestates may be selected so as to produce different sets of colors,including black and white, as desired.

In one embodiment, light enters the modulator 12 through the substrate20 and is output to a viewing position 141. In another embodiment, thestack of layers illustrated in FIG. 10 is reversed, with layer 144closest to the substrate 20 rather than layer 102. In certain suchembodiments, the modulator 12 may be viewed through the opposite side ofthe stack from the substrate 20 rather than through the substrate 20. Inone such embodiment, a layer of silicon dioxide is formed on the ITOlayer 102 to electrically isolate the ITO layer 102.

As noted above, having a separate state for outputting white light in amodulator 140 decouples the selection of the properties of the modulatorcontrolling color saturation from the properties affecting thebrightness of white output. The distance and other characteristics ofthe modulator 140 may thus be selected to provide a highly saturatedcolor without affecting the white light produced in the first state. Forexample, in an exemplary color display, one or more of the red, green,and blue modulators 12 may be formed with optical path lengths Lcorresponding to a higher order of interference.

The modulator 140 may be formed using lithographic techniques known inthe art, and such as described above with reference to the modulator 12.For example, the fixed mirror 16 may be formed by depositing one or morelayers of chromium onto the substantially transparent substrate 20. Theelectrode 102 may be formed by depositing one or more layers of atransparent conductor such as ITO onto the substrate 20. The conductorlayers are patterned into parallel strips, and may form columns ofelectrodes. The movable mirror 14 may be formed as a series of parallelstrips of a deposited metal layer or layers (orthogonal to the columnelectrodes 102) deposited on top of posts 18 and an interveningsacrificial material deposited between the posts 18. Vias through one ormore of the layers described above may be provided so that etchant gas,such as xenon diflouride, can reach the sacrificial layers. When thesacrificial material is etched away, the deformable metal layers areseparated from the fixed layers by an air gap. A highly conductive andreflective material such as aluminum may be used for the deformablelayers, and these strips may form row electrodes in a display device.The conductor 142 may be formed by depositing posts 18 a, over themovable mirror 14, depositing an intervening sacrificial materialbetween the posts 18 a, depositing one or more layers of a conductorsuch as aluminum on top of the posts 18 a, and depositing a conductivelayer over the sacrificial material. When the sacrificial material isetched away, the conductive layer can serve as the electrode 142 whichis separated from the mirror 14 by a second air gap. Each of the airgaps provides a cavity in which the mirror 14 may move to achieve eachof the states described above.

As further illustrated in FIG. 10, in the exemplary modulator 140, theconductive mirror 14 is connected to the row driver 24 of the arraycontroller 22. In the exemplary modulator 140, the conductors 102 and142 are connected to separate columns in the column driver 26. In oneembodiment, the state of the modulator 140 is selected by applying theappropriate voltage potential differences between the mirror 14 and thecolumn conductors 102 and 142 according to the method described withreference to FIGS. 3 and 4.

FIGS. 11A-11C illustrates another exemplary interferometric modulator150 that provides more than two states. In the exemplary modulator 150,the mirror 16 includes both a reflective layer and a conductive layer soas to perform the function of the electrode 102 of FIG. 10. Theconductive layer 142 can also be protected by a second dielectric layer144 a and supported by a support surface 148 that is maintained somedistance above the movable mirror 14 through a second set of supports 18a.

FIG. 11A illustrates the undriven state of the modulator 150. As withthe modulator 140 of FIG. 10, the mirror 14 of the exemplary modulator150 of FIGS. 11A-11C is deflectable towards the dielectric layer 104(e.g., downwards), as in the driven state illustrated FIG. 11B, and isdeflectable in the reverse or opposite direction (e.g., upwards), asillustrated in FIG. 11C. This “upwardly” deflected state may be calledthe “reverse driven state.”

As will be appreciated by one of skill in the art, this reverse drivenstate can be achieved in a number of ways. In one embodiment, thereverse driven state is achieved through the use of an additional chargeplate or conductive layer 142 that can electrostatically pull the mirror16 in the upward direction, as depicted in FIG. 11C. The exemplarymodulator 150 includes what is basically two interferometric modulatorspositioned symmetrically around a single movable mirror 14. Thisconfiguration allows each of the conductive layer of the mirror 16 andthe conductive layer 142 to attract the mirror 14 in oppositedirections.

In certain embodiments, the additional conductive layer 142 may beuseful as an electrode in overcoming stictional forces (static friction)that may develop when the mirror 14 comes in close proximity, orcontacts, the dielectric layer 104. These forces can include van derWaals or electrostatic forces, as well as other possibilities asappreciated by one of skill in the art. In one embodiment, a voltagepulse applied to the conductive layer of the mirror 16 may send themovable mirror 14 into the “normal” driven state of FIG. 11B. Similarly,the next voltage pulse can be applied to the conductive layer 142 toattract the movable mirror 14 away from the mirror 16. In certainembodiments, such a voltage pulse applied to the conductive layer 142can be used to accelerate the recovery of the movable mirror 14 back tothe undriven state illustrated in FIG. 11A from the driven stateillustrated in FIG. 11B by driving the movable mirror 14 towards thereverse driven state. Thus, in certain embodiments, the modulator 150may operate in only two states, the undriven state of FIG. 11A and thedriven state of FIG. 11B, and can employ the conductive layer 142 as anelectrode to help overcome stictional forces. In one embodiment, theconductive layer 142 may be driven as described above each time that themodulator 150 changes from the driven position of FIG. 11C to theundriven position of FIG. 11A.

As will be appreciated by one of skill in the art, not all of theseelements will be required in every embodiment. For example, if theprecise relative amount of upward deflection (e.g., as shown in FIG.11C) is not relevant in the operation of such embodiments, then theconductive layer 142 can be positioned at various distances from themovable mirror 14. Thus, there may be no need for support elements 18 a,the dielectric layer 144 a, or a separate support surface 148. In theseembodiments, it is not necessarily important how far upward the movablemirror 14 deflects, but rather that the conductive layer 142 ispositioned to attract the mirror 14 at the appropriate time, such as tounstick the modulator 12. In other embodiments, the position of themovable mirror 14 as shown in FIG. 11C, may result in altered anddesirable optical characteristics for the interferometric modulator. Inthese embodiments, the precise distance of deflection of the movablemirror 14 in the upward direction can be relevant in improving the imagequality of the device.

As will be appreciated by one of skill in the art, the materials used toproduce the layers 142, 144 a, and support surface 148 need not besimilar to the materials used to produce the corresponding layers 16,105 and 20. For example, light need not pass through the layer 148.Additionally, if the conductive layer 142 is positioned beyond the reachof the movable mirror 14 in its deformed upward position, then themodulator 150 may not include the dielectric layer 144 a. Additionally,the voltages applied to the conductive layer 142 and the movable mirror14 can be accordingly different based on the above differences.

As will be appreciated by one of skill in the art, the voltage appliedto drive the movable mirror 14 from the driven state of FIG. 11B, backto the undriven state of FIG. 11A, may be different than that requiredto drive the movable mirror 14 from the undriven state of FIG. 11A tothe upward or reverse driven state of FIG. 11C, as the distance betweenthe conductive layer 142 and movable mirror 14 may be different in thetwo states. Such requirements can depend upon the desired applicationand amounts of deflection, and can be determined by one of skill in theart in view of the present disclosure.

In some embodiments, the amount of force or duration that a force isapplied between the conductive layer 142 and the movable mirror 14 issuch that it only increases the rate at which the interferometricmodulator transitions between the driven state and the undriven state.Since the movable mirror 14 can be attracted to either conductive layer142 or the conductive mirror 16, which are located on opposite sides ofmovable mirror 14, a very brief driving force can be provided to weakenthe interaction of movable mirror 14 with the opposite layer. Forexample, as the movable mirror 14 is driven to interact with fixedconductive mirror 16, a pulse of energy to the opposite conductive layer142 can be used to weaken the interaction of the movable mirror 14 andthe fixed mirror 16, thereby make it easier for the movable mirror 14 tomove to the undriven state.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. As will be recognized, the present invention may be embodiedwithin a form that does not provide all of the features and benefits setforth herein, as some features may be used or practiced separately fromothers. The scope of the invention is indicated by the appended claimsrather than by the foregoing description. All changes which come withinthe meaning and range of equivalency of the claims are to be embracedwithin their scope.

1. A light modulator, comprising: a first reflector; a first electrodepositioned at a distance from the first reflector; and a secondreflector positioned between the first reflector and the firstelectrode, the second reflector being movable between an undrivenposition, a first driven position, and a second driven position, whereinthe first driven position is closer to the first reflector than is theundriven position and wherein the second driven position is farther fromthe first reflector than is the undriven position; wherein the firstreflector comprises a second electrode configured to drive the secondreflector towards the first driven position, and wherein the firstelectrode is configured to drive the second reflector towards the seconddriven position.
 2. The modulator of claim 1, wherein the firstreflector is at least partially transparent.
 3. The modulator of claim1, wherein the first reflector comprises at least one layer ofreflective material.
 4. The modulator of claim 1, wherein the secondreflector comprises a third electrode.
 5. The modulator of claim 4,wherein each of the second electrode and the third electrode comprises alayer of conductive material.
 6. The modulator of claim 4, wherein thesecond reflector moves to the first driven position in response to avoltage potential applied between the second electrode and the thirdelectrode.
 7. The modulator of claim 6, wherein the modulator reflectswhite light when the second reflector is in the first driven position.8. The modulator of claim 4, wherein the second reflector moves to thesecond driven position in response to a voltage potential appliedbetween the first electrode and the third electrode.
 9. The modulator ofclaim 8, wherein the modulator selectively reflects light in a range ofvisible wavelengths associated with a color when the second reflector isin the second driven position.
 10. The modulator of claim 1, wherein themodulator substantially absorbs incident visible light when the secondreflector is in the undriven position.
 11. A method of fabricating amultistate light modulator, comprising: forming a first reflector;forming a first electrode positioned at a distance from the firstreflector; and forming a second reflector positioned between the firstreflector and the first electrode, the second reflector movable betweenan undriven position, a first driven position, and a second drivenposition, wherein the first driven position is closer to the firstreflector than is the undriven position and wherein the second drivenposition is farther from the first reflector than is the undrivenposition; wherein forming the first reflector comprises forming a secondelectrode configured to drive the second reflector towards the firstdriven position, and wherein the first electrode is configured to drivethe second reflector towards the second driven position.
 12. The methodof claim 11, wherein forming the second reflector comprises forming athird electrode.
 13. The modulator of claim 12, wherein forming each ofthe second electrode and the third electrode comprises forming a layerof conductive material.
 14. A light modulator formed by the method ofclaim
 11. 15. A display comprising a plurality of display elements, eachof the display elements comprising: a first reflective member; a firstconductive member positioned at a distance from the first reflectivemember; and a second reflective member positioned between the firstreflective member and the first conductive member, the second reflectivemember being movable between an undriven position, a first drivenposition, and a second driven position, wherein the first drivenposition is closer to the first reflective member than is the undrivenposition and wherein the second driven position is farther from thefirst reflective member than is the undriven position; wherein the firstreflective member comprises a second conductive member configured todrive the second reflective member towards the first driven position,and wherein the first conductive member is configured to drive thesecond reflective member towards the second driven position.
 16. Thedisplay of claim 15, wherein the first reflective member is at leastpartially transparent.
 17. The display of claim 15, further comprising asubstantially transparent substrate which transmits incident light tothe second reflective member.
 18. The display of claim 15, wherein thefirst reflective member comprises at least one layer of reflectivematerial.
 19. The display of claim 15, wherein the second reflectivemember comprises at least one layer of reflective material.
 20. Thedisplay of claim 15, wherein the second reflective member comprises athird conductive member.
 21. The display of claim 20, wherein each ofthe second conductive member and the third conductive member comprises alayer of conductive material.
 22. The display of claim 20, wherein thesecond reflective member moves to the first driven position in responseto a voltage potential applied between the second conductive member andthe third conductive member.
 23. The display of claim 22, wherein eachdisplay element reflects white light when the second reflective memberof the display element is in the first driven position.
 24. The displayof claim 20, wherein the second reflective member moves to the seconddriven position in response to a voltage potential applied between thefirst conductive member and the third conductive member.
 25. The displayof claim 24, wherein each display element selectively reflects light ina range of visible wavelengths associated with a respective color whenthe second reflective member of the display element is in the seconddriven position.
 26. The display of claim 15, wherein each displayelement substantially absorbs incident visible light when the secondreflective member of the display element is in the undriven position.